When water comes in intimate contact with elemental phosphorus, the elemental phosphorus becomes a constituent of the water in one or more of the following forms.
1. Elemental phosphorus dissolves in water. When water is saturated with elemental phosphorus at room temperature the water will contain about 30 parts per billion of elemental phosphorus. PA1 2. Colloidal phosphorus particles become suspended in water. PA1 3. Settleable phosphorus particles become suspended in water. PA1 Yellow phosphorus was lethal to seawater-maintained brook trout and smelt at concentrations as low as 0.5 .mu.g/liter. Trout that were exposed to low concentrations of yellow phosphorus (0.5 and 7.0 .mu.g/liter) for 50 or more hours turned red and showed signs of extensive hemolysis. At death, all trout that had been exposed to 104 .mu.g/liter yellow phosphorus and lower had hematocrits that were significantly lower than those of the controls. PA1 1. To provide a process for the disposal of phossy water when the phossy water contains little or no settleable particles of elemental phosphorus. PA1 2. To provide a process for the recovery of phossy water when the phossy water contains dissolved elemental phosphorus, colloidal particles of elemental phosphorus, and settleable particles of elemental phosphorus.
Water that contains elemental phosphorus in any one of the three forms in commonly called "phossy water."
It is not feasible to avoid the generation of phossy water during the manufacture of elemental phosphorus, during its storage, and during its processing to make phosphorus chemicals. Nevertheless, elemental phosphorus is a highly toxic chemical, and release of phossy water as a liquid waste is a serious threat to marine life.
Late in 1969 a number of massive fishkills occurred in Long Harbour and neighboring regions of Placentia Bay in Newfoundland. The fishkills were attributed to the startup of a phosphorus producing plant at Long Harbour. A study was undertaken by the Fisheries Research Board of Canada to determine the toxicity of yellow phosphorus to marine life. Results of the research were reported in a book entitled "Effects of Elemental Phosphorus on Marine Life," compiled and edited by P. J. Jangaard, Circular No. 2, November 1972, Atlantic Regional Office, Research and Development, Fisheries Research Board of Canada, Halifax, Nova Scotia.
The book is a compilation of technical papers which describe the pollution problem at Long Harbour, give results of research on toxicity of elemental phosphorus and describe methods used to clean up Placentia Bay. An abstract of one of the technical papers is given below as an example to show the relative sensitivity of marine life to small concentrations of elemental phosphorus in water. The paper abstracted is "Yellow Phosphorus Pollution: Its Toxicity to Seawater-Maintained Brook Trout (Salvelinus fontinalis) and smelt (Osmerus mordax)," by G. L. Fletcher, R. J. Hoyle, and D. A. Horne, Fisheries Research Board of Canada, Halifax Laboratory, Halifax, Nova Scotia.
Other species of marine life exhibited similar sensitivity to very small concentrations of elemental phosphorus in water. It is evident that wastes containing any measurable concentration of elemental phosphorus may be a threat to the environment. And by means of chromotography, elemental phosphorus can be detected in water at concentrations as low as 0.5 .mu.g/liter (0.5 part per billion).
Elemental phosphorus was produced commercially at a federal facility now known as the National Fertilizer and Environmental Research Center (NFERC) at Muscle Shoals, Ala., NFERC is operated by the Tennessee Valley Authority (TVA). Production of elemental phosphorus began at NFERC in 1934 and its production was discontinued in 1976.
Much water is used in the manufacture of elemental phosphorus and this results in the generation of phossy water. Elemental phosphorus is produced by smelting a mixture of phosphate ore, reducing carbon, and silica in electric furnaces. The element leaves the furnace in a gas mixture. The gases are then cooled by contacting them with a circulating stream of water,aand elemental phosphorus condenses as a liquid. From this contact between water and elemental phosphorus the water will contain elemental phosphorus in all three forms discussed above.
Part of the elemental phosphorus is condensed as an emulsion called phosphorus sludge which is comprised of dirt, water, and droplets of liquid phosphorus. Although the composition of phosphorus sludge varies widely, the freshly formed emulsion contains approximately equal percentages of dirt, water, and elemental phosphorus. Processes are available for separating elemental phosphorus from impurities in phosphorus sludge, but it has not been feasible to remove all the phosphorus from the dirt and water. Residue from the various recovery processes contains enough elemental phosphorus for the residue to be a toxic waste. Water is used in the recovery processes and phossy water is generated. For example, elemental phosphorus may be separated from the impurities by distillation in which case phosphorus vapor and water vapor will be condensed by contacting gases with water.
Gases from the electric furnace contain particulates. Electrostatic precipitators were used at NFERC to remove the particulates and thereby decrease the quantity of phosphorus sludge that was made. However, particulates were not completely removed from the gas, and the quantity of phosphorus sludge formed during the condensation of elemental phosphorus was reduced but was not eliminated. Particulates removed from the furnace gas are called precipitator dust.
Precipitator dust is comprised of very small particles. Furnace gases, including elemental phosphorus, adsorb on the surfaces of the particulates and the precipitator dust thereby contains elemental phosphorus.
When elemental phosphorus was being produced at NFERC, phossy water and phosphorus sludge were discharged into a 14-acre settling pond. FIG. 1 is an aerial view of the settling pond. Also, phosphorus-containing wastes were deposited in other ponds, in sumps, and in tanks. Elemental phosphorus was used as a munition during World War II and during the Korean armed conflict. Munitions-grade elemental phosphorus called for almost complete removal of phosphorus sludge from the element, and this was achieved by washing phosphorus with hot water in a vertical tank. The lower density phosphorus sludge floated on top of the liquid phosphorus and separated by overflowing the tank. The phosphorus sludge contained a substantial amount of elemental phosphorus. During wartime emergencies, production of munitions-grade elemental phosphorus took precedence over phosphorus recovery efficiency. The washings containing elemental phosphorus, phosphorus sludge, and phossy water were discharged into the settling pond, along with phossy water from the phosphorus condensers.
Phossy water was clarified in the pond to separate suspended phosphorus particles. Also, phossy water was diluted with cooling water. After settling and dilution, phossy water was discharged into a stream (Pond Creek) as a waste. The average elemental phosphorus content of the waste phossy water was about 90 parts per billion parts of water, but the waste frequently contained higher concentrations.
The quantity of phosphorus sludge deposited in the settling pond (FIG. 1) increased rapidly when phosphorus was being washed. The pond was not lined with an impervious membrane and leakages caused fishkills. In 1980 about half of the 14-acre settling pond was filled in with ash and phosphate ore as shown in FIG. 2. The quantity of elemental phosphorus buried in the pond is not known with accuracy. It is assumed about 3 percent of the elemental phosphorus produced was lost in the phosphorus sludge. Since about 1.1 million tons of elemental phosphorus was produced, elemental phosphorus in sludge will be as follows: ##EQU1## However, the quantity may be greater than 34,000 tons.
FIG. 3 shows the part of the settling pond that was not filled in. The liquid in the unfilled-in part of the pond is phossy water containing dissolved elemental phosphorus, colloidal phosphorus particles, and possibly some settleable phosphorus particles.
The original volume of the 14-acre settling pond was 2,300,000 cubic feet as reported in "Waste Effluent; Treatment and Reuse," Chemical Engineering Progress, volume 65, June 1969. Under the assumption that half the pond was unfilled-in, the volume of phossy water is estimated to be 1,150,000 cubic feet.
Precipitator dust was generated at NFERC at a rate of 0.06 ton (dry basis) per ton elemental phosphorus produced. Since over 1.1 million tons of phosphorus was produced at NFERC, simple calculations indicate about 68,000 tons of precipitator dust was generated. However, precipitators were installed about four years after production of elemental phosphorus was undertaken. Some precipitator dust was distributed as a fertilizer during World War II. Too, small particles comprising precipitator dust adsorb water in outside storage and this increases the quantity. Taking these factors into consideration, it was estimated about 34,000 tons of precipitator dust (wet basis) is stored at NFERC.
When precipitator dust was collected the average elemental phosphorus content was about 0.3 percent. But the elemental phosphorus content may be as high as 2.1 percent.
Processes have been invented to recover phosphorus sludge by a combination of distillation and recycle. The phosphorus sludge is distilled to recover most of the elemental phosphorus. Phossy water will be generated when the phosphorus is condensed. In accordance with U.S. Pat. No. 4,608,241, residue from distillation may be agglomerated by tumbling with a binder formed by reacting acidic phosphorus compounds with alkaline substances to prepare feedstock for phosphorus furnaces. When the feedstock is smelted, elemental phosphorus is produced.
U.S. Pat. No. 4,968,499 discloses a process for converting precipitator dust into phosphorus furnace feedstock whereby the waste is agglomerated by a process similar to that used to agglomerate residue from distillation of phosphorus sludge. Elemental phosphorus present in the precipitator dust will be vaporized when agglomerates are dried. Scrubbing the gas with water for pollution abatement will condense the phosphorus and this will generate phossy water.
Processes were invented to recover phossy water generated during condensation of elemental phosphorus as disclosed in U.S. Pat. Nos. 4,383,847; 4,451,277; and 4,537,615. Phossy water is used instead of process water to manufacture fluid fertilizers.
A process is needed to dispose of the large volume of phossy water shown in FIG. 3. The quantity is too much for recovery in fluid fertilizers as disclosed in U.S. Pat. Nos. 4,383,847; 4,451,277; and 4,537,615. Since the elemental phosphorus content is largely limited to dissolved phosphorus and colloidal phosphorus, evaporation to dryness appeared to be the preferred disposal method. And a suitable source of heat energy is needed to evaporate the phossy water.
Phossy water generated when phosphorus is condensed contains dissolved phosphorus, colloidal phosphorus particles, and settleable phosphorus particles. Although phossy water may be clarified to remove settleable phosphorus particles, industrial clarification processes do not remove all suspended particles. Phosphoric acid accumulates in water recirculated at phosphorus condensers, and the phosphoric acid is neutralized with ammonia forming ammonium phosphate. Thus phossy water bled off from the recirculating stream of condenser water contains the nutrients, nitrogen and P.sub.2 P.sub.5. The nutrients are recovered when the phossy water is added to fluid fertilizers as described in U.S. Pat. Nos. 4,383,847; 4,451,277; and 4,537,615.
Phossy water from phosphorus condensing contains ammonium fluosilicate, potassium fluosilicate, and sodium fluosilicate. Although the concentration of the various salts, including ammonium phosphate, can be increased by recirculating condenser water, fluosilicate salts precipitate as scales on heat transfer surfaces, in pumps, and in piping. Thus the concentration of the salts has to be limited to prevent precipitation of fluosilicates. A phosphorus condensing system is needed which will permit salts in the recirculating condenser water to be concentrated.